54 BULLETIN 
clay should be shaken down dry with a harrow and in no case 
wetted and rolled until the whole mass is well mixed. If large quan- 
tities of clay are to be added it is better to place it in a layer between 
a first and second layer of the cleaner gravel. The foundation then 
can be wetted and rolled without preliminary harrowing, and the 
clay will not stick to the roller wheels. TTet clay in contact with the 
roller can almost never be mixed into the gravel, and endless trouble 
results from permitting it to occur. 
Gravel foundations vary in compacted thickness from about -1 
inches up to about 10 inches, depending on the condition of the sub- 
grade and the character of the road crust, as well as somewhat on 
the quality of gravel used. The most usual thickness is from 5 to 
7 inches. Except that where the road crust is to be of some highly 
improved type a thickness of about 8 inches generally is used. In 
general, foundations of more than about 5 inches in thickness must 
be constructed in two or more courses, as otherwise, unless the per- 
centage of clay is unwarrantably high, it is almost impossible to 
produce a thorough bond between the particles by harrowing and 
rolling. Figure 14 shows a typical cross section for a gravel foun- 
dation upon which the limiting dimensions are indicated. This 
cross section shows the sub grade and foundation crowned to con- 
form with the surface of the finished road, and probably is more 
frequently employed than any other form, but in some cases the sub- 
grade is made flat, or even slightly * ; dished." and the whole crown 
of the road made up either in the foundation or the road crust. 
This latter arrangement provides for additional thickness of the 
pavement near the center where it usually is desirable to have the 
greatest strength. The principal precautions to observe in con- 
structing gravel foundations may be described briefly and com- 
mented upon as follows : 
1. The gravel should be delivered on the work in wagons or cars 
especially adapted to spreading each load uniformly over that part 
of the subgrade for which it is intended. Where loads are dumped 
in one spot and spread later with shovels, as is done frequently, it is 
almost impossible to secure uniform density of the foundation by 
subsequent harrowing and rolling. The spots where the loads are 
dumped will nearly always be more densely compacted than the areas 
in between and. as a result, uneven settlement soon will develop. 
2. "Where it is necessary to spread the gravel in more than one 
course the thickness of the different courses should be approximately 
the same, except that the first course may be made somewhat thicker 
than the others because, in general, a thicker layer of gravel may be 
compacted on the subgrade than when spread over another layer of 
gravel, although compacted. It usually is impracticable to have 
the finished thickness of any course greater than about 5 inches, and 
